Lac Operon: A group of genes in E. coli responsible for metabolizing lactose.
When lactose is present, it binds to the repressor protein, causing it to release from the operator site, allowing RNA polymerase to transcribe the genes needed to break down lactose.
When lactose is absent, the repressor binds to the operator, blocking transcription.
2. Positive vs. Negative Control of the Lac Operon
Negative Control: Involves the repressor protein that binds to the operator to prevent transcription. This happens when lactose is not present.
Positive Control: Involves the cAMP-CAP complex, which enhances the binding of RNA polymerase to the promoter, increasing transcription when glucose levels are low and lactose is present.
3. Regulation of Gene Expression in Prokaryotes vs. Eukaryotes
Prokaryotes: Gene regulation is mainly at the transcriptional level, often through operons like the lac operon, allowing for quick responses to environmental changes.
Eukaryotes: Gene regulation is more complex, involving multiple levels (transcriptional, post-transcriptional, translational, post-translational) and allowing for more precise control in different tissues and developmental stages.
4. Factors Affecting Eukaryotic Gene Expression
Accessibility of Promoter Region: Chromatin structure (e.g., DNA tightly packed in heterochromatin vs. loosely packed in euchromatin) affects whether transcription machinery can access DNA.
Cis-acting DNA Elements & Trans-acting Proteins:
Enhancers/Repressors: DNA sequences that increase or decrease transcription when specific proteins bind to them.
Trans-acting Proteins: Proteins (e.g., transcription factors) that bind to enhancers or repressors to regulate gene expression.
Tissue-specific Transcription Factors: Proteins that activate or repress gene expression only in specific cell types, allowing for tissue-specific functions.
Hormone-specific Activation: Hormones can bind to receptors that act as transcription factors, turning on specific genes in response to hormonal signals.
Alternative RNA Processing: Different ways of splicing RNA transcripts can produce different proteins from the same gene.
RNA Editing: Post-transcriptional modifications of RNA sequences can alter the protein that is produced.
RNAi (RNA Interference): Small RNA molecules can bind to mRNA
